1. Field of the Invention
The invention relates to temperature sensing, or thermostatic, switches used for process control or as over-temperature protective devices; and more particularly to such switches adapted for mounting directly on a printed wiring or circuit board.
Advances in recent years in electronic circuits, and especially miniaturization through the use of integrated circuits which may be mounted on a wiring board or substrate, have created a great need for miniaturized transducers. At the same time, pressures for cost reduction of certain of these transducers have increased markedly. For example, at one time most thermostats were switches of a size that would customarily be picked up by an assembler or a repairman, pushed into or held on a mounting location, and attached by screws or clamps. Electrical connections were then made by attaching a flexible wire, from an external wiring harness, to a screw terminal or the like on the thermostat; or by taking an insulated flexible lead wire extending from the thermostat, and connecting it to a screw or soldered connection point. Thus the cost of installation was itself high, and the cost of a thermostat having many large parts, precisely fitted together, did not seem excessive by comparison.
In recent years automatic mounting of electronic components has become a reality, so that the cost of installation itself, on a per component basis, is relatively small. As a result much effort has been expended in the development of miniature switches and other control components and transducers. Further, it is desirable that the components be mounted mechanically by the same leads or pins which provide electrical connection. This enables installation by inserting pins through holes in a printed wiring board, arranged at standardized spacings, and then finally connected by wave soldering or some similar technique; and still more recently by surface mounting, in which contact surfaces on the outside of the unit, or short lead legs extending from the unit, rest against an electrical contact pad on the surface of a wiring board or substrate, and are directly soldered or otherwise mechanically and electrically connected to that surface, for example by the use of a conducting cement.
2. Description of the Prior Art
Where long contact life, or precise control, are desired in an electrical thermostat, for many years the preferred practice has been to use a bistable bimetallic element as the temperature sensing device. Originally such bistable elements were normally formed as a disc, which would snap relatively violently from being convex in one direction to convex in the other. When popped in a first condition, the center of the disc would bear against an element or linkage which would cause a moveable contact to be pressed, against spring force, into one switch or contact position. Upon occurrence of the predetermined temperature change, "popping" of the bimetal element to the other stable or second condition would remove the operating pressure from the linkage to the contact arm, permitting the contact arm to move to the other switch position. Thermostatic switches of this general construction could be made quite reliably and, in some instances, quite economically. However, they tend to suffer the disadvantage that, for a given size bimetallic disc diameter, only a relatively small distance of motion between the two conditions can be obtained. Therefore, unless a complicated linkage is provided to the switch contacts, only a small separation between the movable and fixed contacts could be achieved when the switch was open. As a result the voltage rating of these switches was relatively limited. Further, in some instances a relatively complex linkage was required in order to provide a good wiping action on the contacts themselves.
In order to provide a greater length of stroke from a given maximum dimension of the bistable bimetallic element, elongated bimetallic elements have been developed which offer a substantial increase in stroke when compared with a disc having a diameter equal to the length of the elongated element. Such a switch is shown in U.S. Pat. No. 4,317,097. This patent also shows an advantageous feature, in which the bimetallic element is not rigidly fixed in place at any point. This has the potential of greatly simplifying manufacture and assembly. However, the structure shown in this patent is large for a given voltage rating, and has the practical disadvantage of requiring a relatively large number of parts. Further, because of the way the bimetallic element is captured, there is no convenient way to adjust the temperature at which the switch operates, after it has been assembled.
Yet another approach to miniaturized thermostats is shown by the article "Resettable Breakers, Thermal Switches Fit DIPs," in Design News, Mar. 18, 1985, page 142. The thermostats described there are in a form which is similar to that of a dual in-line package, but are still relatively large. They appear to use a linear bimetallic spring connected through an over-center linkage to a switch contact.